1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly to a method and an apparatus for transmitting/receiving signals in a multiple-input multiple-output (MIMO) communication system provided with a plurality of antenna elements.
2. Background of the Related Art
Generally, the multiple-input multiple-output (MIMO) system has been evolved from a conventional single-input single-output (SISO) communication system and a single-input multiple-output (SIMO) communication system, and developed so that it is used in a technique that requires a high-capacity data transmission. This MIMO system transmits information through M antennas and receives the information through N antennas, and is considered as an essential element in the fourth-generation communication system that requires a high efficiency of frequency.
FIG. 1 is a diagram illustrating an example of a conventional MIMO system.
Referring to FIG. 1, a transmitter includes a transmission multiple-input multiple-output processor 101 for dividing an information source (or bit stream or data stream) into M sub-bit streams for signal processing, and a modulator 102 for modulating processed signals and applying modulated signals to M antennas. A receiver includes a demodulator 103 for receiving the signals transmitted from the transmitter through N antennas and demodulating the received signals, and a reception multiple-input multiple-output processor 104 for processing and restoring demodulated signals to the original bit stream.
FIG. 2 is a diagram illustrating an example of a transmitter of a conventional D-BLAST MIMO system. FIG. 3 is a diagram illustrating an example of a receiver of a conventional D-BLAST MIMO system.
According to the MIMO system of FIGS. 2 and 3, the distance between antennas is 1.5λ, and four transmission and reception antennas are used. The MIMO system of FIGS. 2 and 3 is of a diagonal-bell labs' layered space-time (D-BLAST) type.
Referring to FIGS. 2 and 3, the bit stream to be transmitted is divided into M sub-bit streams of the same ratio by a demultiplexer 203, and the sub-bit streams are encoded in a signal processor 202. The encoded signals are periodically connected to the respective antennas by a tetrad cyclic shifter 201 to be transmitted to the receiver.
The signal processor 202 generates transmission signals having different transmission delays by modulating and encoding the respective bit streams of one information source, and applies the signals to the respective antennas. The signals transmitted through the antennas include the symbols encoded from the same information.
The cyclic shifter 201 periodically changes the connection of the transmission antennas to the signals processed from the sub-streams. For each τ seconds, the connection between the processed signals and the transmission antennas is periodically changed. This enables the transmitter to use a delay diversity technique, and M processed signals on fading channels are all received through the N receiving antennas. Herein, since the M transmitted signals are received through all the receiving antennas, any one of the transmitted signals is received without being seriously affected by the worst channel environment when it passes through the multiple-paths environment.
Accordingly, the symbols which the received signals include are diagonally detected in respective space layers (discriminated by the receiving antennas) of a detector 301. That is, the desired symbol values are detected through cancellation of previously detected symbols from the received signals and nullification of the non-detected symbols. This process diagonally detects the desired symbol values as many as the number of antennas.
The nullification enables the detection of the strongest signal by removing other weak signals, and the cancellation enables the detection of the remaining weak signals by removing the previously detected signals from the original received signals.
Then, the detected symbols for each antenna are collected by a multiplexer 302, and generated as one data stream in the respective space layer, and the data streams of all the antennas are finally combined in the maximum ratio by a combiner 303.
This maximum ratio combining type is for making the value of an output signal-to-interference ratio maximum by applying the respective channels to gains in proportion to square roots of the signal-to-interference ratios in the respective channels. These signal-to-interference ratios in the respective channels are added together to provide a whole signal-to-interference ratio.
FIG. 4 is a view illustrating another example of a transmitter of a conventional V-BLAST MIMO system. FIG. 5 is a view illustrating an example of a receiver for receiving signals from the transmitter illustrated in FIG. 4.
According to the MIMO system of FIGS. 4 and 5, the distance between antennas is 4λ, and four transmission and reception antennas are used. The MIMO system of FIGS. 4 and 5 is of a vertical-bell labs' layered space-time (V-BLAST) type.
The MIMO system of FIGS. 4 and 5 has a similar construction to that of FIGS. 2 and 3.
However, a signal processor 401 in FIG. 4 simplifies the decoding process by simply performing a vector encoding process for changing a bit to a symbol. The signal processor 401 modulates and encodes a plurality of sub-data streams divided from one information source. That is, M encoded symbols divided from the same information are transmitted through a plurality of antennas, respectively.
Also, a detector 501 arranges the M received symbols in the order of their levels of the signal-to-interference ratio, and detects a desired symbol from the symbols having a good receiving condition among the arranged symbols.
The frequency efficiency of the system used in FIGS. 4 and 5 is lower than that of the MIMO system used in FIGS. 2 and 3, but it can be implemented using a more simplified receiving circuit.
That is, since the transmitted symbols in FIGS. 2 and 3 may suffer a fatal error as passing through a multi-paths environment, an obstacle may arise when the receiver obtains the original information stream from the received symbols. Also, the construction of the receiver is complicated, and the channel coding is applied in a limited manner. However, the efficiency of the frequency use is heightened.
On the contrary, the system as shown in FIGS. 4 and 5 can be implemented using a simplified receiving circuit as an alternative, but the efficiency of the frequency use is degraded, and the distance between antenna elements should be widened. This is because the distance 10λ should be put between the antennas for no correlation between the antennas, and for practical use, the distance of at least 4λ should be put between the antennas for no correlation (which corresponds to 80% of the case having no correlation).
Accordingly, it is required to design a MIMO system that improves the advantage of the D-BLAST type system illustrated in FIGS. 2 and 3, and makes up for the weak points in the V-BLAST type system illustrated in FIGS. 4 and 5.